Voltammetric analytical techniques are used to determine the antioxidant concentrations of a wide range of lubricating oils and greases as described in U.S. Pat. Nos. 4,744,870 and 4,764,258. U.S. Pat. No. 4,744,870 describes a cyclic voltammetric procedure for measuring the secondary aromatic amine antioxidant concentrations of ester based lubricating oils used in aircraft engines. U.S. Pat. No. 4,764,258 describes a linear sweep voltammetric procedure for measuring the concentrations of different type antioxidants such as sterically hindered phenols, alkyl amines, and zinc dialkyldithiophosphates (ZDDP) in petroleum based lubricating oils and greases. The linear sweep voltammetry is used in place of the cyclic voltammetry for the petroleum based oils because ZDDP, naturally occurring phenols, and other electrochemically active species cause electrode filming during the reductive cycle of the cyclic voltammetry analysis.
Although the voltammetric techniques are capable of differentiating between different classes of antioxidants (e.g., hindered phenols versus secondary aromatic amines), they are incapable of differentiating between different types of antioxidants of the same class (one peak produced by mixture of different amines). Until recently, the inability to distinguish between different amine antioxidants with similar structures was not a limitation since the different antioxidants and their oxidation products were highly soluble in the basestock used in the specific lubricating oil formulations.
However, environmental regulations and higher equipment operating temperatures have required changes in the basestock refining processes and traditional lubricant formulations. Specifically, highly refined petroleum basestocks with reduced sulfur and nitrogen contents and synthetic hydrocarbons are now being used in many lubricating oil formulations due to their superior thermal and oxidative stabilities compared to traditional petroleum basestocks. Also, secondary aromatic amine antioxidants are now being used in the refined petroleum and synthetic hydrocarbon basestocks of industrial turbine oils due to their higher temperature capabilities and in the basestocks of automotive and diesel engine oils to supplement the reduced levels of ZDDP.
In many of the applications, reports of lubricant related equipment damage and failures coincided with the introduction of the new oil formulations utilizing high purity basestocks and secondary amine antioxidants. In many instances, the insolubilities of the basestock oxidation products (varnish, unsaturated carboxylic acids) and secondary amine antioxidant by-products (sludge, aromatic amine dimers and trimers) were identified as the cause(s) of the lubricant related malfunctions. In addition to the solubility issues, the secondary aromatic amines are less effective antioxidants at lower temperatures than hindered phenols leading to increased rates of basestock oxidation, and consequently, increased amounts of varnish adhering to or clogging components with reduced clearances such as servo valves, bearings, filters, etc.
Previous research projects with cyclic voltammetry (see U.S. Pat. No. 4,744,870 and original RULLET Part III paper in Lub. Eng. Kauffman, R. E., (1990), “Development of a Remaining Useful Life of a Lubricant Evaluation Technique. Part III. Cyclic Voltammetric Techniques,” Lubr. Eng., 46, 1, pp 709-716) have shown that the tendencies of secondary aromatic amines to form insoluble dimers and trimers (sludge) are strongly related to chemical structure. When they are the sole antioxidant used in the oil formulation, sterically hindered aromatic amines such as p,p′-dioctyldiphenyl amine (DODPA) undergo reversible electro-oxidation/reduction during cyclic voltammetric analysis; successive oxidation/reduction cycles do not affect the composition of the DODPA molecule at the molecule surface (i.e. less likely to form sludge). In contrast, less hindered aromatic amines such as phenyl alpha naphthyl amine (PANA) and p-octylphenyl alpha naphthyl amine (Octyl-PANA) undergo irreversible electro-oxidation/chemical reactions to produce lower solubility aromatic amine polymers (sludge) which then undergo further electro-oxidation/reduction reactions. When DODPA and PANA combinations are used, irreversible electro-oxidation reactions occur to produce DODPA-PANA polymeric compounds (sludge).
Since the capability of a secondary aromatic amine antioxidant to undergo irreversible electro-oxidation during cyclic voltammetry is related to the antioxidant's chemical structure and tendency to form sludge during use in operating equipment, cyclic voltammetry potentially has both the capability of determining the individual secondary aromatic amine concentrations as well as the sludge tendencies of in-service oils. However, hindered (antioxidants) and unhindered (naturally occurring) phenols, ZDDP, and other compounds in the lubricating oil undergo irreversible electro-oxidation reactions forming insoluble polymer films on the electrode surface during cyclic voltammetric analyses making the technique impractical for fully formulated lubricating oils.
Thus, there exists a need for a voltammetric method by which to distinguish among different types of aromatic amine antioxidants of the same class.